Surface Mounted Technology (SMT) has been increasingly replacing through-hole mounting systems as component size reduction drives technological development. SMT involves use of flat pads on a surface of a printed wiring board (PWB), application of solder paste to the pads via a template, and application of components to the pads, wherein the components have leads which match the pads. The board may then be reflowed and the components soldered to the pads.
Most assembly failure occurs at interface points, namely within solder joints. For this reason, testing to determine life expectancy of an assembly has included analysis of thermal cycling on solder joints. As a result of testing involving thermal cycling, an expected lifetime of a device can be determined as well as identification of possible failure modes that may be corrected in order to extend the lifetime of the device.
Currently, thermal cycling of SMT solder joints includes immersing a PWB in an environment, such that heat is either absorbed or lost by the PWB. Typically, this is done by using oven-like chambers, which may perform temperature cycling in either a single chamber or dual chambers. In single chamber cycling, air within the chamber is incrementally heated. A rate of heating is known as a ramp rate. Once a desired ambient temperature is reached, the ambient temperature is stabilized while a temperature of the PWB lags due to thermal transference. The temperature within the chamber may then be dropped by applying a coolant or by a lack of heating.
In dual chamber thermal cycling, each chamber is regulated at a respective constant temperature and the PWB is physically moved from one chamber to another chamber in a process known as thermal shock. The process of thermal shock may include use of a gas, such as air or nitrogen, or liquids, such as fluoropolymers.
The current methods for performing thermal cycling for SMT connections has many disadvantages, such as, for example, cost, size and equipment complexity. Often, each item of equipment must be purchased separately, at great cost, and an end-user must custom design a configuration that meets space requirements and end-user requirements. Such a system tends to be unreliable, resulting in downtime. Although single chamber systems are less complex and more reliable than dual chamber systems, single chamber systems take up considerable space and are expensive to maintain.
In addition, both single chamber systems and dual chamber systems operate by heating an intermediate medium, such as air. In order to accommodate various sizes of devices, chambers are built with an excess of volume. Thus, for example, in order to heat a small device for testing, energy must be expended to heat an entire volume of a chamber.